Abstract
AbstractTiny samples of ancient atmosphere in air bubbles within ice cores contain argon (Ar), which can be used to reconstruct past temperature changes. At a sufficient depth, the air bubbles are compressed by the overburden pressure under low temperature and transform into air-hydrate crystals. While the oxygen (O2) and nitrogen (N2) molecules have indeed been identified in the air-hydrate crystals with Raman spectroscopy, direct observational knowledge of the distribution of Ar at depth within ice sheet and its enclathration has been lacking. In this study, we applied scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) to five air-hydrate crystals in the Greenland NEEM ice core, finding them to contain Ar and N. Given that Ar cannot be detected by Raman spectroscopy, the method commonly used for O2 and N2, the SEM-EDS measurement method may become increasingly useful for measuring inert gases in deep ice cores.
Highlights
The pores in snow contain atmospheric air that transforms to preserved-air bubbles when the pores close off
By analyzing the Ar signal from energy-dispersive X-ray spectroscopy (EDS) of the airhydrate crystals, we argue that Ar exists in the air-hydrate crystals of deep ice cores
Since the observed ice core samples were from depths below bubble-to-air hydrate transition zone (BHTZ) (Fig. 1), air inclusions other than air-hydrate crystals were formed after the ice core recovery
Summary
The pores in snow contain atmospheric air that transforms to preserved-air bubbles when the pores close off These air bubbles in an ice core are the only known paleoenvironmental archive of the actual ancient atmosphere with a time axis in the depth direction. Summer insolation may influence certain physical properties of snow that control the magnitude of close-off fractionation (Bender, 2002; Kawamura and others, 2007; Fujita and others, 2009). Greenhouse gases such as carbon dioxide (CO2) and methane (CH4) are trace components in the atmosphere, and their past atmospheric concentrations can only be known from the analysis of deep ice cores. The 40Ar/38Ar ratio can be used for dating very old ice, based on the increasing rate of atmospheric 40Ar over a million-year timescale (Bender and others, 2008)
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